1. Field of the Invention
This invention relates to minimizing brightness reversion of wood pulps bleached with certain bleaching agents and especially relates to minimizing such reversion of relatively impure wood pulps such as mechanical pulps used for newsprint and magazines.
2. Review of the Prior Art
Wood pulps can be classified as chemical pulps and low-cellulose pulps. Chemical pulps are prepared by pulping methods involving chemical removal of much of the non-cellulosic wood materials, such as lignins, hemicelluloses, and other impurities, to leave a relatively purified pulp composed of 80-100% cellulose. These pulping methods involve chemical digestion and are typified by kraft, sulfite, and like methods.
The low-cellulose wood pulps are prepared by mechanical and chemi-mechanical methods which leave the bulk of the non-cellulosic constituents of the wood in the pulp because separation of the wood material into fibers operates primarily through mechanical attrition of the wood, either in the form of chips or as logs, so that the bulk of the non-cellulosic constituents thereof remain in the fibers. Such fibers contain up to about 60% cellulose, with the remaining elements in the pulp being on the order of 40-60% non-cellulosic wood materials.
The strictly mechanical methods for producing low-cellulose wood pulps are represented by the groundwood process, in which logs or other large pieces of wood are ground on a grinding stone, and by various refining processes in which the wood chips are mechanically subdivided within disc or similar refiners. Chemi-mechanical methods involve softening of the wood with aqueous softening agents, such as sulfites, bisulfites, and the like, without substantial extraction of non-cellulosics before the wood is mechanically subdivided into fibers.
The low-cellulose wood pulps are particularly desired because of their low cost and generally satisfactory physical properties. Their preparation involves very little loss of the original wood, and methods of producing them are generally less expensive than methods involving strong chemical attack. The particular utility of these pulps is in the preparation of printing papers, newsprint, molded products, corrugated paper, boxboards, and the like.
While it is economically favorable to retain the bulk of the wood materials in mechanical pulps, it causes the pulps to have a relatively dark color and makes it difficult to bleach them.
Wood is a very complex and irregular macromolecular system made of carbohydrates and phenylpropane units as the two main components. The former are primarily celluloses but include various amounts of hemicelluloses, and the latter are generally identified as lignin. Within these essentially colorless materials, there are relatively small amounts of chromophoric groups in the lignin which cause the yellow-brown color of wood. The basic chromophoric structures which have been identified are of three main types: (a) ortho- and para-quinones, (b) ortho-hydroxy- and para-hydroxy-phenyl ketones, and (c) para-quinone methides.
When a mechanical pulp is bleached with certain bleaching processes, such as the chlorine process, these chromophores are absolutely destroyed and yield is markedly reduced. Chlorine "bleaching" is actually a lignin-destructive process that is consequently used only on high-cellulose pulps, such as kraft pulp; it produces lignin byproducts which are subsequently removed by alkaline extraction.
Other bleaching processes, however, such as reductive bleaching processes and the peroxide process, result in retention of substantial amounts of potential chromophores. Reductive bleaching methods, which are less expensive than oxidative bleaching methods, are generally used for low-cellulose pulps, employing dithionites, borohydrides, thiourea dioxide, and the like. Oxidative bleaching with hydrogen peroxide generally produces higher brightness than reductive bleaching but is more expensive.
Mechanically-disintegrated wood pulp, commonly referred to as groundwood pulp, is usually bleached with a reductive agent. For example, at least 0.25% by weight of sodium dithionite is used, based on the weight of the dry pulp, and preferably 0.5% to 1.5% on the same basis. Other compounds, such as sodium tripolyphosphate, disodium phosphate, and the like, are also generally added to improve the bleaching process. Trisodium nitrilotriacetate (NTA) and trisodium ethylenediamine-tetraacetate (EDTA) are also widely employed to chelate the metal ions.
Varied ions in a groundwood bleach system are suspected of causing major problems which seriously affect the brightness gain in a given pulp bleaching operation. First, the groundwood pulp acts as a low-capacity cation exchanger with a great affinity for metals, such as iron, which, in turn, produce colored compounds in the pulp. Second, during storage of the bleached pulp, a brightness reversion, which requires an increased quantity of the bleaching agent to be used initially, is likely to develop. Third, metal ions in the bleaching system may catalyze the decomposition of sodium dithionite, which causes further increased consumption thereof to obtain a given pulp brightness.
Brightness is one of the most important product specifications in making paper. There is increasing pressure on paper manufacturers to improve brightness of their products, even though the raw material brightness may be lower for their groundwood pulps. These pressures have pushed dithionite bleaching technology to its limits. Consequently, many paper manufacturers are considering switching to the more expensive oxidative bleaching processes in order to meet the increasing brightness demand.
Even with the best bleaching methods, however, bleached pulps tend to revert toward their original dark color on standing, particularly when in wet pulp form, and overcoming such color reversion is a major objective of the pulp and paper industry.
Brightness increases for groundwood pulps, when bleached with dithionite, occur through the reduction of certain chromophores to lower their absorption in the visible portion of the electromagnetic spectrum. Measurement of the brightness change by such a treatment, relative to a magnesium oxide or a barium sulfate standard, shows that less than 1% sodium dithionite applied to the pulp can yield a change of 12-16 percentage points of brightness. In practice, this is never achieved because a reduction of the chromophores and the consequent color removal are not permanent. Oxidative reversion of these sites occurs especially while the pulp is wet and exposed to air, and only 8-10 points of brightness gain can be retained.
It has previously been noted that ascorbic acid can be used as a bleaching agent and also as a uv-light screen. In these capacities, ascorbic acid increases brightness through reduction of chromophores or by itself undergoing light-induced decomposition instead of chromophores in the pulp.
A study reported in the Pulp and Paper Magazine of Canada, 65 T459-T466(1964), involved treating samples of filter paper with solutions of model compounds and then heating the treated paper at various temperatures and for various time periods in a circulating oven. Of the 34 model compounds tested, glucoronic acid gave the most reversion on oven aging at 105.degree. C. and ascorbic acid and triose uronic acid were the worst at 75.degree. C.
Wood veneers treated with aqueous solutions of ascorbic acid or its mixture with NaCl have improved resistance to discoloration by light, as reported in Jpn. Kokai Tokkyo Koho, JP 57,159,604.
U.S. Pat. No. 3,829,358 discloses the addition of a diglycolate and particularly the alkali metal salts of diglycolic acid, which are nitrogen and phosphorus-free, as adequate chelating agents for groundwood pulp bleaching with sodium dithionite. It particularly teaches that disodium diglycolate has a calcium sequestering power which is stronger than sodium citrate on a weight basis and a weaker power than either EDTA or NTA.
Thiourea is employed to minimize color reversion in reductive bleaching processes, as taught in U.S. Pat. No. 3,507,743, for pulps from a wide variety of wood sources while using dithionites, borohydrides, thiourea dioxide, and the like.
U.S. Pat. No. 4,238,282 discloses a method for increasing the final brightness of pulp contaminated with iron or manganese during a chlorine bleaching process having one or more alkaline stages. The process particularly comprises treating the pulp with at least 0.1 pound per ton of a water-soluble chelating agent by adding it under acidic conditions, after the last alkaline stage has extracted almost all of the oxidized lignin, to prevent iron and manganese from interfering with the final brightness of the pulp after this oxidative bleaching is completed. After such chlorine treatment, substantially no chromophores remain; the only source of color is metal ion contamination. Ascorbic acid, tartaric acid, and gluconic acid are among the chelating compositions that were evaluated. When added to the D-stage make-up water at a level of 4 pounds/ton of the neat product, the brightness preservation for ascorbic acid was 75.0%.